The present invention provides recombinant methods and materials for producing polyketides by recombinant DNA technology. More specifically, it relates to narbonolides and derivatives thereof. The invention relates to the fields of agriculture, animal husbandry, chemistry, medicinal chemistry, medicine, molecular biology, pharmacology, and veterinary technology.
Polyketides represent a large family of diverse compounds synthesized from 2-carbon units through a series of condensations and subsequent modifications. Polyketides occur in many types of organisms, including fungi and mycelial bacteria, in particular, the actinomycetes. There is a wide variety of polyketide structures, and the class of polyketides encompasses numerous compounds with diverse activities. Tetracycline, erythromycin, FK506, FK520, narbomycin, picromycin, rapamycin, spinocyn, and tylosin, are examples of such compounds. Given the difficulty in producing polyketide compounds by traditional chemical methodology, and the typically low production of polyketides in wild-type cells, there has been considerable interest in finding improved or alternate means to produce polyketide compounds. See PCT publication Nos. WO 93/13663; WO 95/08548; WO 96/40968; WO 97/02358; and WO 98/27203; U.S. Pat. Nos. 4,874,748; 5,063,155; 5,098,837; 5,149,639; 5,672,491; and 5,712,146; Fu et al., 1994, Biochemistry 33: 9321-9326; McDaniel et al., 1993, Science 262: 1546-1550; and Rohr, 1995, Angew. Chem. Int. Ed. Engl. 34(8): 881-888, each of which is incorporated herein by reference.
Polyketides are synthesized in nature by polyketide synthase (PKS) enzymes. These enzymes, which are complexes of multiple large proteins, are similar to the synthases that catalyze condensation of 2-carbon units in the biosynthesis of fatty acids. PKS enzymes are encoded by PKS genes that usually consist of three or more open reading frames (ORFs). Two major types of PKS enzymes are known; these differ in their composition and mode of synthesis. These two major types of PKS enzymes are commonly referred to as Type I or xe2x80x9cmodularxe2x80x9d and Type II xe2x80x9citerativexe2x80x9d PKS enzymes.
Modular PKSs are responsible for producing a large number of 12, 14, and 16-membered macrolide antibiotics including methymycin, erythromycin, narbomycin, picromycin, and tylosin. These large multifunctional enzymes ( greater than 300,000 kDa) catalyze the biosynthesis of polyketide macrolactones through multistep pathways involving decarboxylative condensations between acyl thioesters followed by cycles of varying xcex2-carbon processing activities (see O""Hagan, D. The polyketide metabolites; E. Horwood: New York, 1991, incorporated herein by reference). The modular PKS are generally encoded in multiple ORFs. Each ORF typically comprises two or more xe2x80x9cmodulesxe2x80x9d of ketosynthase activity, each module of which consists of at least two (if a loading module) and more typically three or more enzymatic activities or xe2x80x9cdomains.xe2x80x9d
During the past half decade, the study of modular PKS function and specificity has been greatly facilitated by the plasmid-based Streptomyces coelicolor expression system developed with the 6-deoxyerythronolide B (6-dEB) synthase (DEBS) genes (see Kao et al., 1994, Science, 265: 509-512, McDaniel et al., 1993, Science 262: 1546-1557, and U.S. Pat. Nos. 5,672,491 and 5,712,146, each of which is incorporated herein by reference). The advantages to this plasmid-based genetic system for DEBS were that it overcame the tedious and limited techniques for manipulating the natural DEBS host organism, Saccharopolyspora erythraea, allowed more facile construction of recombinant PKSs, and reduced the complexity of PKS analysis by providing a xe2x80x9ccleanxe2x80x9d host background. This system also expedited construction of the first combinatorial modular polyketide library in Streptomyces (see PCT publication No. WO 98/49315, incorporated herein by reference).
The ability to control aspects of polyketide biosynthesis, such as monomer selection and degree of xcex2-carbon processing, by genetic manipulation of PKSs has stimulated great interest in the combinatorial engineering of novel antibiotics (see Hutchinson, 1998, Curr. Opin. Microbiol. 31: 319-329; Carreras and Santi, 1998, Curr. Opin. Biotech. 9: 403-411; and U.S. Pat. Nos. 5,712,146 and 5,672,491, each of which is incorporated herein by reference). This interest has resulted in the cloning, analysis, and manipulation by recombinant DNA technology of genes that encode PKS enzymes. The resulting technology allows one to manipulate a known PKS gene cluster either to produce the polyketide synthesized by that PKS at higher levels than occur in nature or in hosts that otherwise do not produce the polyketide. The technology also allows one to produce molecules that are structurally related to, but distinct from, the polyketides produced from known PKS gene clusters. It has been possible to manipulate modular PKS genes other than the narbonolide PKS using generally known recombinant techniques to obtain altered and hybrid forms. See, e.g., U.S. Pat. Nos. 5,672,491 and 5,712,146 and PCT publication No. WO 98/49315. See Lau et al., 1999, xe2x80x9cDissecting the role of acyltransferase domains of modular polyketide synthases in the choice and stereochemical fate of extender unitsxe2x80x9d Biochemistry 38(5): 1643-1651, and Gokhale et al., Apr. 16, 1999, Dissecting and Exploiting Intermodular Communication in Polyketide Synthasesxe2x80x9d, Science 284: 482-485.
The present invention provides methods and reagents relating to the modular PKS gene cluster for the polyketide antibiotics known as narbomycin and picromycin. Narbomycin is produced in Streptomyces narbonensis, and both narbomycin and picromycin are produced in S. venezuelae. These species are unique among macrolide producing organisms in that they produce, in addition to the 14-membered macrolides narbomycin and picromycin (picromycin is shown in FIG. 1, compound 1), the 12-membered macrolides neomethymycin and methymycin (methymycin is shown in FIG. 1, compound 2). Narbomycin differs from picromycin only by lacking the hydroxyl at position 12. Based on the structural similarities between picromycin and methymycin, it was speculated that methymycin would result from premature cyclization of a hexaketide intermediate in the picromycin pathway.
Glycosylation of the C5 hydroxyl group of the polyketide precursor, narbonolide, is achieved through an endogenous desosaminyl transferase to produce narbomycin. In Streptomyces venezuelae, narbomycin is then converted to picromycin by the endogenously produced narbomycin hydroxylase. (See FIG. 1) Thus, as in the case of other macrolide antibiotics, the macrolide product of the narbonolide PKS is further modified by hydroxylation and glycosylation. FIG. 1 also shows the metabolic relationships of the compounds discussed above.
Picromycin (FIG. 1, compound 1) is of particular interest because of its close structural relationship to ketolide compounds (e.g. HMR 3004, FIG. 1, compound 3). The ketolides are a new class of semi-synthetic macrolides with activity against pathogens resistant to erythromycin (see Agouridas et al., 1998, J. Med. Chem. 41: 4080-4100, incorporated herein by reference). Thus, genetic systems that allow rapid engineering of the narbonolide PKS would be valuable for creating novel ketolide analogs for pharmaceutical applications. Furthermore, the production of picromycin as well as novel compounds with useful activity could be accomplished if the heterologous expression of the narbonolide PKS in Streptomyces lividans and other host cells were possible. The present invention meets these and other needs.
The present invention provides recombinant methods and materials for expressing PKSs derived in whole and in part from the narbonolide PKS and other genes involved in narbomycin and picromycin biosynthesis in recombinant host cells. The invention also provides the polyketides derived from the narbonolide PKS. The invention provides the complete PKS gene cluster that ultimately results, in Streptomyces venezuelae, in the production of picromycin. The ketolide product of this PKS is narbonolide. Narbonolide is glycosylated to obtain narbomycin and then hydroxylated at C12 to obtain picromycin. The enzymes responsible for the glycosylation and hydroxylation are also provided in recombinant form by the invention.
Thus, in one embodiment, the invention is directed to recombinant materials that contain nucleotide sequences encoding at least one domain, module, or protein encoded by a narbonolide PKS gene. The recombinant materials may be xe2x80x9cisolated.xe2x80x9d The invention also provides recombinant materials useful for conversion of ketolides to antibiotics. These materials include recombinant DNA compounds that encode the C12hydroxylase (the picK gene), the desosamine biosynthesis and desosaminyl transferase enzymes, and the beta-glucosidase enzyme involved in picromycin biosynthesis in S. venezuelae and the recombinant proteins that can be produced from these nucleic acids in the recombinant host cells of the invention.
In one embodiment, the invention provides a recombinant expression system that comprises a heterologous promoter positioned to drive expression of the narbonolide PKS, including a xe2x80x9chybridxe2x80x9d narbonolide PKS. In a preferred embodiment, the promoter is derived from a PKS gene. In a related embodiment, the invention provides recombinant host cells comprising the vector that produces narbonolide. In a preferred embodiment, the host cell is Streptomyces lividans or S. coelicolor. 
In another embodiment, the invention provides a recombinant expression system that comprises the desosamine biosynthetic genes as well as the desosaminyl transferase gene. In a related embodiment, the invention provides recombinant host cells comprising a vector that produces the desosamine biosynthetic gene products and desosaminyl transferase gene product. In a preferred embodiment, the host cell is Streptomyces lividans or S. coelicolor. 
In another embodiment, the invention provides a method for desosaninylating polyketide compounds in recombinant host cells, which method comprises expressing the PKS for the polyketide and the desosaminyl transferase and desosamine biosynthetic genes in a host cell. In a preferred embodiment, the host cell expresses a beta-glucosidase gene as well. This preferred method is especially advantageous when producing desosaminylated polyketides in Streptomyces host cells, because such host cells typically glucosylate desosamine residues of polyketides, which can decrease desired activity, such as antibiotic activity. By coexpression of beta-glucosidase, the glucose residue is removed from the polyketide.
In another embodiment, the invention provides the picK hydroxylase gene in recombinant form and methods for hydroxylating polyketides with the recombinant gene product. The invention also provides polyketides thus produced and the antibiotics or other useful compounds derived therefrom.
In another embodiment, the invention provides a recombinant expression system that comprises a promoter positioned to drive expression of a xe2x80x9chybridxe2x80x9d PKS comprising all or part of the narbonolide PKS and at least a part of a second PKS, or comprising a narbonolide PKS modified by deletions, insertions and/or substitutions. In a related embodiment, the invention provides recombinant host cells comprising the vector that produces the hybrid PKS and its corresponding polyketide. In a preferred embodiment, the host cell is Streptomyces lividans or S. coelicolor. 
In a related embodiment, the invention provides recombinant materials for the production of libraries of polyketides wherein the polyketide members of the library are synthesized by hybrid PKS enzymes of the invention. The resulting polyketides can be further modified to convert them to other useful compounds, such as antibiotics, typically through hydroxylation and/or glycosylation. Modified macrolides provided by the invention that are useful intermediates in the preparation of antibiotics are of particular benefit.
In another related embodiment, the invention provides a method to prepare a nucleic acid that encodes a modified PKS, which method comprises using the narbonolide PKS encoding sequence as a scaffold and modifying the portions of the nucleotide sequence that encode enzymatic activities, either by mutagenesis, inactivation. insertion, or replacement. The thus modified narbonolide PKS encoding nucleotide sequence can then be expressed in a suitable host cell and the cell employed to produce a polyketide different from that produced by the narbonolide PKS. In addition, portions of the narbonolide PKS coding sequence can be inserted into other PKS coding sequences to modify the products thereof The narbonolide PKS can itself be manipulated, for example, by fusing two or more of its open reading frames, particularly those for extender modules 5 and 6, to make more efficient the production of 14-membered as opposed to 12-membered macrolides.
In another related embodiment, the invention is directed to a multiplicity of cell colonies, constituting a library of colonies, wherein each colony of the library contains an expression vector for the production of a modular PKS derived in whole or in part from the narbonolide PKS. Thus, at least a portion of the modular PKS is identical to that found in the PKS that produces narbonolide and is identifiable as such. The derived portion can be prepared synthetically or directly from DNA derived from organisms that produce narbonolide. In addition, the invention provides methods to screen the resulting polyketide and antibiotic libraries.
The invention also provides novelpolyketides and antibiotics or other useful compounds derived therefrom. The compounds of the invention can be used in the manufacture of another compound. In a preferred embodiment, the antibiotic compounds of the invention are formulated in a mixture or solution for administration to an animal or human.
These and other embodiments of the invention are described in more detail in the following description, the examples, and claims set forth below.